The catalytic cracking or steam cracking gasolines are excellent basic constituents for producing commercial motor gasolines, owing to their high content of olefinic compounds and aromatic compounds which provide for a high clear octane number of these gasolines. Commonly the sulfur content of these gasolines (which may be defined as the fraction distilling between 30.degree. and 220.degree. C. for example) depends on the sulfur content of the heavy charge subjected to catalytic cracking. Up to now the sulfur content of these fractions was lower than those of the trade specifications, after admixture with gasolines obtained by other processes as, for example, catalytic reforming. A sweetening treatment of these gasolines was performed for removing compounds of the mercaptan type, which have a substantial corrosion effect and reduce the favourable effect, on the octane number, of lead additives.
This conventional treatment does not change substantially the total sulfur content of said gasoline.
Presently the increase of the sulfur content of the catalytic cracking or steam cracking charges and the decrease of the tolerable sulfur content of motor gasolines in the trade, give a further interest to a desulfurization treatment of these gasolines which removes the sulfur without changing to a substantial extent the octane number of these gasolines.
Different solutions have been proposed in order to obtain this result. According to U.S. Pat. No. 4,132,632 the entire cut 61.degree.-197.degree. C. (ASTM distillation) is treated on a catalyst comprising molybdenum oxide and cobalt oxide deposited on a carrier containing at least 70% of magnesia. The contents of active metals, calculated as oxides, are limited to the range from 0.5 to 2% for the cobalt family and from 4 to 6% for the molybdenum family. The ratios CoO/MoO.sub.3 in the catalysts mentioned in the examples are those of the conventional hydrodesulfurization catalysts.
These proportions are defined by the ratio: ##EQU1## wherein the active elements are expressed in gram-atoms, said ratio R being about 0.39 in the examples.
According to the U.S. Pat. No. 4,140,626 the total cut is also treated on a catalyst containing at least 70% of magnesia as MgO. The cobalt oxide and molybdenum oxide contents are such that the ratio R=CoO/CoO+MoO.sub.3 of the prepared catalysts is close to 0.26 as in the conventional catalysts. The examples show that the catalysts prepared with alumina carriers are less active than those prepared with magnesia carriers and deactivate during time.
In the U.S. patent application Ser. No. 152,215 filed on May 25, 1980, now U.S. Pat. No. 4,334,982, it has been shown that the total cut may be treated on a hydrodesulfurization catalyst containing (a) a non-acid carrier selected from the group consisting of silica, aluminas of specific surface lower than 70 m.sup.2 /g, of the .alpha. type or tetragonal .gamma. type, cobalt, nickel, barium, magnesium and calcium aluminates carriers of a specific surface lower than 130 m.sup.2 /g, and autoclaved aluminas of specific surface lower than 135 m.sup.2 /g, and (b) at least cobalt and tungsten and/or cobalt and molybdenum, the total weight of cobalt and tungsten or of cobalt and molybdenum, expressed as oxide, being from 2 to 30% with respect to the total catalyst mass, the atomic ratio ##EQU2## being higher than 0.55 (the metal concentrations being expressed in gram-atoms).